1. Field of the Invention
The present invention generally relates to a N-channel TFT, in particular, to an N-channel TFT having unique structure and OLED display apparatus and electronic device using the same.
2. Description of Related Art
Being self-luminous, OLEDs eliminate the need for a backlight that is necessary in liquid crystal display devices (LCDs), and thus they are most suitable when manufacturing thinner devices. Also, the self-luminous OLEDs are high in visibility and have no limit in terms of viewing angle. These are the reasons for the attention that light emitting devices using the OLEDs are receiving in recent years as display devices to replace CRTs and LCDs.
Driving circuits for an OLED display apparatus can be categorized into two kinds, i.e., voltage driving circuits and current driving circuits. However, no matter which kind of driving circuits is used, a thin-film-transistor (TFT) is used as a driving transistor for adjusting OLED current provided to the OLED. Refer to FIG. 1, a circuitry diagram of a conventional OLED pixel is provided. As shown in FIG. 1, the gate terminal of the P-channel TFT 12 is controlled by a control signal passed through the transistor 100 and stored in capacitor 102 for adjusting the OLED current provided to the OLED 14.
Because the luminance of OLED completely depends on the threshold voltage and mobility of the P-channel TFT 14, the display image uniformity is poor even with a little variation of TFT characteristics.
To improve image uniformity, prior arts use N-channel TFT to replace the P-channel TFT for reducing the affection caused by the threshold voltage variation as shown in FIG. 2. Because the current flow through OLED depends on Vgs and Vth of driving N-TFT as well as the voltage across OLED, the negative feedback effect makes current flow through OLED more uniformity, therefore results in better image uniformity. However, the hot-carrier effect makes the N-channel TFT 22 unreliable and therefore the lifetime of the whole circuit is shortened.